The present invention relates generally to vertical cavity surface emitting lasers (VCSELs) and methods for forming the same. More particularly, the present invention relates to a method and apparatus for the in-situ control of epitaxial layer thickness and deposition temperature during the formation of long wavelength VCSELs.
VCSELs are highly favored as optical sources in today""s optoelectronics industry. An array of VCSELs is simultaneously formed on a substrate such as gallium arsenide (GaAs), by epitaxially forming a multiplicity of films over the GaAs substrate. The formed VCSEL includes a lower mirror or distributed Bragg reflector (DBR), an active region, and an upper mirror or DBR. The active region is formed between the upper and lower DBRs and supplies the photons which provide the light source. The active region may consist of one or more quantum wells, quantum dots, quantum wires, or other electrically or optically pumped gain media. The mirrors disposed above and below the active region cause the light to be reflected within the optical cavity of the VCSEL, which, in turn, stimulates additional photon generation from the active region. It is important to control the operational characteristics of a VCSEL, for example, the electrical current to optical power ratio, the current threshold for lasing, and the wavelength of the light which is internally reflected and emitted.
The composition, film characteristics and film thicknesses of the layers that combine to form the DBRs, determine the wavelength of reflected light. As such, it is important to control these characteristics to produce a mirror or DBR having a desired mirror center wavelength for reflecting light and a desired range of wavelengths which reflect light. Long wavelength VCSELs, for example, are currently being targeted to have a mirror center at about 1300 nanometers (nm) in order to reflect and emit light having a wavelength on the order of 1300 nm. Each of the individual layers that combine to form the mirrors should desirably be formed to a thickness equal to a quarter wavelength of the light that is desired to be reflected, divided by the index of refraction of the material, in order to insure that light of the desired wavelength is emitted. It is thus important to control the thickness of the individual layers. The thicknesses of the individual layers depend directly upon deposition time. It is therefore important to control deposition time.
The individual layers that combine to form the mirrors are epitaxially formed at elevated temperatures, and the film quality of the individual films and therefore the reflectance of the mirror, are dependent upon the temperature of formation. Moreover, at elevated temperatures, the measured DBR qualities such as mirror center are different than at room temperature. A measured mirror center versus temperature correlation is available to determine whether the desired mirror center at room temperature has been achieved for a particular structure. As such, the substrate temperature at which the mirror center measurement is made, should be accurately determined in order to assess whether or not the desired mirror center has been achieved, whether or not a film thickness adjustment must be made, and, if so, to what extent. As such, it is also important and desirable to monitor substrate temperature during formation.
Qualities such as film thickness, film quality, film formation rate, film formation temperature and reflectance of the DBR should desirably be monitored as the DBR is being formed. It is particularly desirable to continuously monitor these qualities at a stage early enough in the deposition process such that a real-time adjustment can be made during the film formation process to produce a final mirror structure having the desired DBR center wavelength and optical range. Existing techniques for layer thickness control of DBRs used in VCSELs include emissivity corrected pyrometric interferometry, reflection spectroscopy, and single wavelength reflection oscillations. Each of these techniques involves the in-situ measurement of a reflection off the substrate surface during the epitaxial formation of the semiconductor film layers. There are at least two shortcomings associated with each of these approaches. First, the optical system should preferably be designed to accommodate the unpredictable and variable deflection of the reflected light caused by substrate wobble during rotation. In the case of reflection corrected pyrometric interferometry and single wavelength reflectance oscillations, a further complication is the necessity of measuring absolute reflectivity. Absolute reflectivity measurements are especially difficult in the presence of substrate wobble. The second shortcoming associated with each of these methods is that the optical viewports used to monitor reflectance become clouded during the epitaxial deposition process and cause errors in the absolute reflectivity measurements by attenuating both the transmitted and reflected optical signals.
It is therefore desirable to provide an apparatus and method for providing in-situ measurement of the reflectivity of the DBR mirrors and substrate temperature, preferably as the layers which make up the DBR mirror are being formed. It is further desirable to provide such a method that preferably avoids and is not susceptible to the shortcomings described above. Moreover, it is desirable to provide such an apparatus and method for obtaining these measurements at a preliminary stage of the process sequence used to form the DBR, so that film formation parameters can be adjusted during the formation process and the DBR being produced, will have desired reflectance qualities.
To meet these and other needs, the present invention provides a method and apparatus for controlling the characteristics of a series of films as they are being formed on a substrate. In one embodiment, the present invention provides an apparatus and method for utilizing the transmission of light through the substrate on which the films are being formed. The light is transmitted through the substrate to provide real-time information on the mirror center of the films as they are being formed. A raw transmission intensity spectrum provides this information. The present invention also provides an apparatus and method for using light reflected from the substrate to normalize the transmission intensity spectrum. The normalized transmission intensity spectrum is preferably used to determine substrate temperature.
In another embodiment, the present invention further provides for analyzing the transmission intensity spectrum data, evaluating the measured mirror center and substrate temperature and preferably providing feedback to the film formation system on a real-time basis in order to adjust system settings and to ensure that the formed series of films has the desired thickness and mirror center and reflects light at the desired wavelength.